As for azo-compounds, it is known that they exhibit a charge-generating function. Because of this, have been expected to be usable as a charge-generating material, and various studies have been made of those azo-compounds. For example, Japanese Laid-open patent application 47-37543 proposes an organic electrophotographic photosensitive member prepared by using an azo-compound.
However, the known azo-compounds are mostly such that are narrow with respect to visible light region or exhibit a strong absorption in near infrared region and do not exhibit a photosensitive wave property in the entire of visible light region.
Those organic electrophotographic photosensitive members prepared by using such azo-compounds are such that are poor in durability since a reduction is caused not only in the photosensitive characteristics but also in the sensitivity upon repeated use.
Now, the so-called Carlson's process has been wildly used in order to obtain copied images using an electrophotographic photosensitive member. The Carlson's process includes charging step wherein the photosensitive member is uniformly sensitized with corona charge, exposure step wherein the sensitized photosensitive member is subjected to exposure of an original image to form a latent image corresponding to the original image on the surface thereof, development step wherein the latent image is developed with a toner developer to form a toner image, transfer step wherein the toner image is transferred onto a transfer sheet such as paper, fixing step wherein the transferred toner image on the transfer sheet is fixed, and cleaning step wherein the residual toner on the photosensitive member is removed. In order to obtain a high quality copied image in the Carlson's process, it is required for the electrophotographic photosensitive member to excel in both charging characteristics and photosensitive characteristics and to be low in residual potential after exposure. As the electrophotographic material which satisfies these requirements, there are known inorganic photoconductors of selenium, cadmium sulfide, etc. However as for such inorganic photoconductors, there are disadvantages that they are noxious and are of a relatively high production cost.
In view of this, instead of those noxious materials, there have been proposed a number of organic electrophotographic photosensitive members prepared using organic materials, for example, as found in the foregoing literature.
An organic electrophotographic photosensitive member has been spotlighted since it excels in both processability and economy and is large in freedom. And various studies have been made thereof. The conventional organic electrophotographic photosensitive member has a photosensitive layer comprising a charge generatiing material capable of generating electric charge upon exposure and a charge transporting material capable of transporting the electric charge generated.
In order for the organic electrophotographic photosensitive member to satisfy the various conditions desired therefor, it is necessary to appropriately select the charge generating material and the charge transporting material.
However, as for the conventional organic electrophotographic photosensitive member in which a known charge generating material is used, there are problems that it is difficult to precisely reproduce a color original in particular depending upon the absorption wave characteristics thereof and the sensitivity as a whole is insufficient.
In view of the above, there is an increased demand for earlier provision of an effective charge generating material which efficiently generates photocarrier upon light irradiation and if possible, also exhibits a function of transporting electron.